The most usual technique for assessing the quality of a carrier gas, such as carbon dioxide, involves the use of specific chromatography equipment, which include various types of detectors to ensure the sensitivity and selectivity of analysis of the habitual contaminants present in carbon dioxide. In addition to being expensive, such equipment has the disadvantage of not permitting continuous monitoring of the gas being used in production. Such pieces of equipment only carry out ad hoc sample analysis. This technique is used habitually in production centers to evaluate the quality of the dioxide obtained, but as such equipment is expensive it can hardly be installed in any plant that consumes carbon dioxide, such as a carbonated drinks bottling plant. One alternative is to take ad hoc samples that can be sent to the pertinent laboratory for analysis. However, this system does not permit continuous monitoring of the gas flow, while the costs involved therein are far from negligible.
Known in the market are analyzing systems for analyzing the quality of carbon dioxide, comprising various types of specialized equipment such as:                sulfur compounds analyzers, generally based on pyro-luminiscence systems;        aromatic hydrocarbon analyzers, based on PID (Photo Ionization Detector) systems with ultraviolet-light lamp;        total hydrocarbons analyzers, based on FID (Flame Ionization Detector) systems.        
Such analysis systems have the disadvantage of being expensive for installing in carbon dioxide consuming plants, while neither do they permit the carrying out of a real-time analysis of a continuous flow of carbon dioxide.
There exists in the market no low-cost system capable of carrying out an (even partial) real-time analysis of the quality of carbon dioxide.
None of the habitual techniques used for evaluating the quality of carbon dioxide is based on the utilization of sensors based on semiconductor-type metal oxides.
Known in the art are gas sensors based on semiconductor-type metal oxides for the detection of gases such as toxic gases in the atmosphere. These are simple, low-cost and robust sensors.
Sensors based on semiconductor-type metal oxides have been developed for the detection of reducing and oxidizing gases in the presence of pure air and, therefore, in the presence of oxygen.
It is known that in the presence of pure air the active material or semiconductor metal oxide (type n), when heated to a temperature between 300° C. and 500° C., adsorbs atmospheric oxygen until it reaches a state of equilibrium. The process of adsorption of an oxygen atom involves the taking up of an electron from the conduction band of the metal oxide. Therefore, when a sensor is in the presence of pure air and in equilibrium, it shows high electrical resistance, also called base resistance.
It is known that if the sensor is exposed to the presence of a reducing gas, the gas will react with the adsorbed oxygen, once again establishing a state of equilibrium. In this case, the concentration of adsorbed oxygen atoms will be lower than that which existed in the presence of pure air, and this will show itself in a larger number of electrons on the conduction band. This results in a very marked reduction of sensor resistance. This effect is reversible, for the sensor can recover its base resistance if it is once again exposed to the presence of pure air.
In the presence of an oxidizing gas, competition arises around the adsorption sites between that gas and the oxygen. This shows itself in new state of equilibrium in which the sensor resistance increases. This effect is in turn reversible.
It is known that the operational principle of the type of sensors described can be summarized in that the conductance of such devices changes progressively with the changes that take place in the composition of the atmosphere.
No sensors are known, however, based on semiconductor-type metal oxides that permit the detection of reducing and oxidizing gases in the complete absence of oxygen in a carrier gas atmosphere or current.